In modern wireless communication systems, such as the code division multiple access (CDMA) system, wideband code division multiple access (WCDMA) system, universal mobile telecommunication system (UMTS), and long term evolution (LTE) system, a variable envelope modulation technology that performs amplitude modulation and phase modulation concurrently is used to fully utilize the spectrum. This technology has a high peak to average power ratio (PAPR) and a wide dynamic range. However, if the PAPR is too high, the transmit end may impose high requirements upon the linearity of the power amplifier and always run under the peak voltage level.
The variable envelope modulation technology needs to amplify signals by using a linear power amplifier. To ensure the linearity, a power backoff method is generally used. However, the power backoff method may reduce the output capacity and power efficiency of the power amplifier. The envelope tracking (ET) technology is one effective method for improving the efficiency of the power amplifier in the case of a high PAPR. By using the ET technology, high power efficiency can always be maintained in the case of peak output power and low output power. The combination of the ET technology and digital pre-distortion (DPD) technology can not only ensure the linearity but also improve the efficiency of the power amplifier. With the development of the multi-carrier technology, the bandwidth of an envelope signal may also reach several tens of MHz. Due to the limiting factors such as the semiconductor technology and switching frequency, the bandwidth of an ordinary switching power supply can hardly meet the requirement of the ET bandwidth. In addition, the output noise and distortion may be modulated to carriers, thus causing spread of the out-of-band spectrum and greatly affecting the adjacent channel power ratio (ACPR) of the signal.
When the output voltage is used for switching directly and the number of voltage levels is associated with the number of control circuits, a substantial number of voltage levels and circuits are required to implement high-accuracy tracking.